This invention relates to a motor for rotating a magnetic disk. In particular, it relates to a motor which is used for an application in which center portions of a few magnetic disks, each normally formed with a magnetic layer on an aluminum disk, are secured to a rotor frame of the motor, and data is recorded on and reproduced from the magnetic disks by magnetic heads disposed near the upper and lower surfaces of each magnetic disk. In addition, the motor is suitable for rotating optical disks and magneto-optical disks.
FIG. 4 is a longitudinal sectional view showing a conventional structure of a motor for rotating and driving a magnetic disk. In FIG. 4, an upright hollow shaft 2 is formed in the center of a circular mounting frame 1 made by aluminum die casting or the like. A motor 3 has a hollow fixed shaft 4 that is pressed onto the upright hollow shaft 2. A laminated core 6 of an armature 5 is pressed and secured to an axial central portion of the hollow fixed shaft 4. Shoulders 7 are defined on both ends of the hollow fixed shaft 4 to form small diameter portions 8 and 9. Lower portion 8 is slightly longer in the axial direction than upper portion 9 in order to accommodate two Belleville springs 10 that are inserted onto the lower small diameter portion 8. Springs 10 exert force on a lower roller bearing 11 that is inserted on portion 8. An upper roller bearing 14 is pressed onto portion 9 and confined between the upper portion 9 and an end plate 13 of a rotor frame 12, so that the frame 12 may be rotatably supported by bearings 11 and 14 on the hollow fixed shaft 4. The rotor frame 12 is in the shape of a cup. An axial cylindrical side wall 54 covers an outer peripheral surface of the armature 5, and an outer end of the side wall 54 serves as a magnetic disk support 15 which is projected outwardly perpendicular to the motor axis. Support 15 is opposed to the mounting frame 1, thereby leaving a small clearance. An annular recess 16 formed at an inner corner of the magnetic disk support 15 is provided for a good contact between the magnetic disk support 15 and a magnetic disk placed thereon.
An annular permanent magnet 17 having the desired number of poles is pressed and secured into the cylindrical side wall 54, after which an end plate 19 is pressed and secured to the roller bearing 11. The roller bearing 11 is then pressed and secured to shaft 4, and the end plate 19 is attached to the end of the side wall 54 to form a rotor 20. The rotor 20, the armature 5 and the hollow fixed shaft 4 comprise the motor 3. A set of lead wires 22 of an armature coil 21 is drawn out of a hole 23 provided in the hollow fixed shaft 4 and pulled into the hollow portion of shaft 4. The wires are then pulled outside of the motor through the hollow portion of the hollow shaft 2.
FIG. 5 is a sectional view of a portion of the conventional magnetic disk drive motor having the structure described above. In FIG. 5, center holes of the magnetic disks 24 are fitted on the side wall 54 of the rotor frame 12, placed on the disk receiving base 15 in a predetermined spaced relation with spacers 25, and secured by means of a pressure plate 26 and a screw 27. In order to maintain a predetermined small clearance between the surface of each magnetic disk 24 and a radially moving magnetic head 28, vibrations resulting from rotation of the side wall 54 and magnetic external disk support 15 have to be minimized. Further, the width of vertical variation of the rotating disk support 15 should be 0.005 mm or less.
However, in the aforementioned conventional construction, it is very difficult to make the lateral hole 23 through which the lead wires 22 pass in the hollow fixed shaft 4, and the number of parts is large, which increases the cost of the motor.